In current OFDMA based wireless communication systems such as IEEE 802.16e-2005, the TDD frame structure shown in FIG. 1 comprises two subframes, one for downlink and the other for uplink. The 802.16e-2005 standard specifies many possible frame durations ranging from 2 ms to 20 ms in length. However, the current WiMAX forum profile (Release 1.0) specifies that only 5 ms frames are used to ensure that all WiMAX forum certified equipment is interoperable. The current standard defines many features which, if combined, may improve the system throughput and ensure that the user experiences the best possible performance. However, in order for the combinations to operate efficiently, the base station (BS) requires knowledge of the propagation channel that the mobile station (MS) experiences. For this reason, the BS allocates the MS with a specific CQICH channel(s) that resides within the CQICH region shown in the uplink subframe in FIG. 1. This allocated CQICH channel is used by the MS to report either physical or effective SINR, one of which will be instructed by the BS. In this case, the MS calculates a channel quality measurement based on the physical or effective CINR which provides information on the actual operating condition of the receiver, including interference and noise levels and signal strength. The information is fed back to the BS via the allocated CQI feedback channel (CQICH) and, as a result, the BS may use the information to manage its radio resources or to perform basic link adaptation for the MS.
Considering the (legacy) IEEE 802.16e-2005 TDD frame structure, the first symbol is occupied by a preamble which is mainly used for synchronisation purposes, but is also used for transmitter identification during network entry and handover procedures. On the second and third symbols following the preamble is the FCH. The FCH is transmitted using a well-known format and provides sufficient information to decode the following MAP message, i.e. the MAP message length, coding scheme and active sub-channels. Following the FCH is the DL-MAP which may be followed by the UL-MAP. These MAP messages provide information on the allocated resource (slots) for traffic and control channels within the frame. These MAPs contain DL-MAP_IEs and UL-MAP_IEs which define bursts within the frames, (i.e. one MAP_IE will be related to one burst within the frame). The information within these MAP_IEs, such as the subchannel offset and symbol offset, is crucial as it is used by the MS to locate the resource within the subframes. For the purposes of the CQICH region, a Fast Feedback IE is transmitted in the UL-MAP which informs the MSs of its location within the UL subframe (i.e. subchannel and symbol offsets). The CQICH region comprises an integer number of slots (one subchannel by three OFDMA symbols), where one slot can be used as a CQICH channel.
The TDD OFDMA frame structure for IEEE 802.16m is illustrated in FIG. 2, which demonstrates how it differs from the legacy frame structure in that a radio frame consists of eight subframes. The 20 ms super frame consists of four equally sized radio frames further comprising eight subframes where each subframe can be allocated to either DL or UL. It may be possible to introduce up to four switching points within one radio frame and, in the context of CQICH feedback, this feature allows for a faster feedback rate if the CQICH BS processing delay is less than four subframes, potentially improving the ability to support accurate link adaptation for high mobility users.
In the current IEEE802.16e-2005 standard, two concurrent CQICH channels can be supported by any MS, where one channel is mainly used for physical CINR reports and the other for effective CINR measurements. It is desirable for the BS to adapt to the best possible physical layer operating mode or efficiently manage its radio resources, employing many features to maximize the system performance such as adaptive MIMO switching, Distributed/Localized switching or FFR, using the two channels per MS. As the radio configuration changes over time for a given MS, a new channel estimation report may be required for the new data zone/subframe/mode in which a data burst may be transmitted (as a result of the physical layer mode change). One solution is to increase the number of available CQICH channels per MS, but this would be at the expense of reducing uplink capacity and thus diminishing the accessible resource for raw data. The BS instructs which type of measurement is to be reported on each CQICH channel via two independent CQICH_Alloc_IEs which will be transmitted within the UL-MAP. These information elements can be sent once to the MS and the MS can periodically (for x frames) report the required measurements on the specified channel indicated by a CQICH_ID.
The BS may instruct the MS to report, for instance, a physical CINR measurement of preamble reuse-1 on the first CQICH channel as this may give the BS enough information from all the MSs to assist the BS in performing a technique known as Fractional Frequency Reuse (FFR). In FFR, the users at the cell/sector edge operate with a fraction of all sub-channels available while the inner cell users operate with all sub-channels available. Usually, the cell edge users are operated with frequency reuse=3 (called R3) while the inner cell users are operated in R1. In frame transmission aspects, the R3 users are grouped into a separate time slot in the frame (called a zone), which is separated in time from the R1 zone. The perceived benefits of FFR lies with the provision of a better signal quality to the cell edge users, through the physical isolation of the interference sources. It is expected that the improved signal quality can also bring higher throughput for the cell edge users. However, this comes at the cost of reduced resource availability.
On the second CQICH channel, the BS may instruct the MS to report an effective CINR measurement (based on pilot or data subcarriers from data block or zone/subframe/mode) as this can be used to perform link adaptation. However, when the MS's preferred zone/subframe/mode changes from R1 to R3 or vice-versa, the MS needs to be instructed to report effective CINR measurement from the new data zone/subframe/mode. This involves additional overhead for de-allocation/re-allocation of the second CQICH channel through the CQICH_Alloc_IE. In addition, it adds to the delay in the availability of correct CQICH measurements for the new data zone/subframe/mode, leading to inaccurate link adaptation. This leads to an increased overhead in the UL-MAP and reduced efficiency in resource management.
FIG. 3 relates to an example of CQICH signalling in the FFR case, where the BS collects all the information required to distribute the users between the Reuse-1 (R1) and Reuse-3 (R3) zones. In this example, it is assumed that a specific MS is allocated in a Reuse-1 zone, and the CQICH channels for this MS are assigned as follows:
CQICH Channel 1 (Physical CINR from Preamble R1)—For zone selection
CQICH Channel 2 (Effective CINR from Pilot/Data Subcarriers of R1 zone)—For Link Adaptation
The Physical CINR report is used by the BS to switch the MS adaptively between R1 and R3 zones, whereas the Effective CINR report is used for performing link adaptation. In FIG. 3, Channel 1 signalling is indicated by hatched blocks and Channel 2 is indicated by non-hatched blocks.
FIG. 3 highlights the scenario where the BS decides to switch a MS (user) from an R1 zone to an R3 zone, with this being referred to as the Radio Configuration Switch Point. The physical layer operating mode of the MS will now change, with the MS being allocated data within the R3 zone. In this case, the BS sends another CQICH_Alloc_IE (related to Channel 2) to inform the MS to measure and report the Effective CINR for the R3 zone to allow for accurate link adaptation (MCS selection).